Solid secondary electrochemical cells, and batteries containing such cells, consist of pairs of electrodes of opposite polarity separated by an electrolyte. The charge flow between electrodes is maintained by an ionically conducting solid electrolyte.
Solid secondary lithium electrochemical batteries comprise a lithium anode and have many advantages over other electrical storage devices. Lithium batteries are capable of much higher power storage densities than batteries not based on the lithium, and have excellent shelf life and cycle life. Such batteries are generally composed of a lithium anode and a composite cathode containing carbon and an intercalating compound such as the oxides or sulfides of vanadium, molybdenum, cobalt, manganese and nickel. The anode and cathode are separated by a solid electrolyte which may be inorganic or organic. Much interest has recently focused on solid polymeric electrolytes composed of a solid polymeric matrix, an inorganic ion salt and an electrolyte solvent or plasticizer. Successful use of lithium batteries depends on their safety during operations under normal conditions and even under abusive usage. An abusive use such as short circuiting or rapid overcharging of the battery may initiate self-heating of the battery, as opposed to merely resistive heating, leading to thermal runaway. The main processes causing self-heating of a secondary lithium cell involve the chemical reaction between cycled lithium and electrolyte. While it was previously believed that the temperature of onset of the first thermal interaction between lithium and electrolyte solvent is near 125.degree. C., it is now known that the reactions are initiated at temperatures near 100.degree. C. At temperatures greater than 100.degree. C., contributions to cell self-heating come from exothermic decomposition of the electrolyte as well as reaction between lithium and the electrolyte salt. See, for example, U. von Sacken and J. R. Dahn, Abstract 54, p. 87, The Electrochem. Soc. Extended Abstracts, Vol. 90-2, Seattle Wash., Oct. 14-19, 1990.
To control overheating under abusive usage, it has been suggested that a thermally activated separator be developed for insertion between the cathode and the anode. It has been suggested many times that a microporous sheet might function as a battery separator if it exhibited low resistivity at normal operating temperatures but irreversibly transformed into a product having high resistivity at high temperatures, while maintaining its overall length, breadth and physical integrity (see U.S. Pat. Nos. 4,731,304; 4,973,532; 4,287,276; 4,361,632; 4,190,707; 4,075,400; and 4,650,730). But microporous polymeric films presently employed as separators in lithium batteries are generally not capable of preventing uncontrolled overheating. In general, polymeric separators disintegrate, to one extent or another, under the influence of heat and thermal reactions, or become dimensionally unstable.
It has been suggested by F. C. Laman, M. A. Gee and J. Denovan, Electrochem. Soc. Letters, 140 (1993) L51, that for a separator to function well as an internal safety device in a lithium battery, it should have the following characteristics: a melting point close to 100.degree. C. (i.e., sufficiently below the initiation of self-heating, but well above operating temperatures that should be tolerated), a high dimensional stability temperature preferably above the melting point of lithium, and a high degree and rate of shutdown, giving rise to an impedance increase of at least three orders of magnitude within a few degrees Celsius in temperature. The authors' reference to dimensional stability, and their conclusion that all these characteristics could be obtained easily only by combining different separators, demonstrates the persistent reliance on physical barriers in battery separator technology.
It would be advantageous to the art of secondary lithium battery design if battery separators were not physical barriers, but were integral to the electrolyte, and, in fact, functioned as the electrolyte up to a specified initiation temperature, and thereafter, the electrolyte underwent an irreversible transformation to function as an ionic insulator at all temperatures greater than the initiation temperature.